In Patent Reference 1, a structure that is provided with a Faraday rotator between a first birefringence unit provided at an input side and a second birefringence unit provided at an output side is publicly known as a polarization non-dependent type optical isolator.
In the optical isolator of Patent Reference 1, ordinary light and extraordinary light that are separated by the first birefringence unit are both focused onto a core end face of an output side optical fiber.
A case in which polarizers are provided at the input side and output side of a Faraday rotator such that only light with a polarization plane in a specified direction is transmitted and light with a polarization plane in another direction is absorbed is publicly known as a polarization dependent type isolator.
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. S57-100410